Errors Cut Safety Margin in Bomb Tests

By MATTHEW L. WALD, Special to the New York Times

Published: February 17, 1989

NEVADA TEST SITE—
Nuclear bomb tests here have for years been conducted with an unacceptably small margin of safety, Government officials say.

They attribute the problem to overestimates of the strength of the rock in which the blasts were set off.

The errors, disclosed in an interview, were discovered after a test in 1986 in which the rock gave way, $32 million in equipment was destroyed and radioactive gases produced by the explosion had to be vented to the atmosphere instead of being sealed underground.

But officials of the Department of Energy said this was one of the few instances in the last 15 years in which radiation was released, and they said the tests had caused no damage to human health. The extent of contamination of the earth here remains unknown, though, and a recent report says further study is needed. Changes in the Tests

In the wake of the new findings on rock strength, the department has made major changes in the way the underground tests are conducted.

The conclusion that operations were conducted here for years with too small a margin for error comes as safety problems are troubling two other Energy Department plants.

In January 1987, Energy Secretary John S. Herrington decided to shut the N Reactor, at the Hanford Nuclear Reservation in Washington State because of inadequate safety equipment. And at the Savannah River Plant, near Aiken, S.C., engineers concluded that they had been operating three reactors for years at a heat too great for the emergency cooling system to handle. All three reactors have been shut since April for safety improvements.

Although most of the blasts here are done to test atomic bombs themselves, some test the effects of explosions on various items. It is in the area of the site where the blast-effects tests are conducted that rock strength was overestimated. Simulated Attack in Space

Engineers realized this after the test named Mighty Oak, on April 10, 1986, in which they tried to simulate the effect that a blast in space would have on an orbiting satellite or other space equipment. To simulate an attack in space they exploded a device at one end of a long, horizontal pipe and allowed the radiation to rush along the pipe toward the target equipment. The pipe is equipped with doors to shield the target from the blast's shock wave, since there would be no shock wave at great distances in space.

The instant the radiation passes the doors, they are supposed to close.

In the test, the shock wave penetrated to the equipment, leading engineers to believe at first that the doors had not closed.

But in an interview in January at the Energy Department's Nevada operations office in Las Vegas, department officials said they had found that the doors had closed. The shock wave was transmitted instead when the surrounding rock gave way. 'Much Closer to the Edge'

''We were much closer to the edge than we realized,'' said James K. Magruder, assistant manager for operations. The department has not yet issued a final report on the test.

Officials said testing procedures had since been changed to make sure radiation does not escape. Stronger barriers are installed, and more of the tunnel in which the pipe is laid is filled in before detonation.

The changes add more than $9 million per test, the officials said. They would not give the total test cost, which they said was highly variable. The annual budget for the site is about $1 billion. About 12 tests are conducted each year.

Nuclear explosions create vast amounts of radiation, some of it emitted by tiny particles that can be absorbed into the body by swallowing or inhaling. Radiation was spread around the globe by atmospheric tests in the 1940's and 50's by the United States in Nevada, the South Pacific and elsewhere and by the Soviet Union and other countries. But this country and the Soviet Union forswore above-ground blasts in the Limited Test Ban Treaty of 1963, and the American record in keeping the radiation underground has been good, especially in the last 15 years. Site Set Up in 1950

The Nevada Test Site, which is northwest of Las Vegas and 1,350 square miles, about a third larger than the state of Rhode Island, was established by President Harry S. Truman in 1950 and was at first used for the atmospheric tests. Since 1963, the site has been used only for underground blasts, and it is now the country's only nuclear bombing range.

The site is the most heavily bombed in this country, possibly in the world. So many nuclear bombs have been exploded here, in fact, that the National Aeronautics and Space Administration brought astronauts here to study craters before it sent them to the moon.

Around the site, there are frequent reminders of its function. The workers' cafeterias are decorated with color photos of bomb craters. On the roads, signs remind drivers to pull over when they see ''special convoys'' with flashing blue lights; those are the ones carrying ''nuclear devices.'' (Engineers here reserve the word ''bomb'' for a nuclear device that is in an aerodynamically shaped casing.) Arid Conditions at Site Although department officials said radiation has rarely escaped into the atmosphere in the last 15 years, a recent survey suggested that much more needed to be learned about underground water, the most likely avenue for the spread of radiation. Bruce W. Church, assistant manager for environment, safety and health, acknowledged this in an interview.

He said that when department advisors visit the site, ''I can predict what they will say: 'You need more wells.' '' But the Energy Department's budget for such research is only about $3 million to $4 million a year. ''It costs us about $1 million to put in a well,'' Mr. Church said, explaining that the cost was high because the water table was so far below the surface.

But a low water table is believed to be an advantage environmentally since explosions above the water table create pollutants that take a long time to reach the water. The area gets only 6 inches of rain a year, limiting the water available to wash pollutants into the soil.

Indeed, a preliminary environmental survey made public by the Energy Department last April found that the extremely arid conditions limited the movement of radiation in water through the underground soil; in some cases, the water would take 1,000 years to leave Government-controlled land.